Acoustic transducer



May 26, 1970 H. o. ROBINSON TA!- 3,513,937

ACOUSTIC TRANSDUCER Filed Sept. 27, 1968 I NVENTORS HARALD D. ROBINSON YSTEPHEN L. MOSHIER ATTQRNEYS United States Patent 3,513,937 ACOUSTIC TRANSDUCER Harald D. Robinson, Arlington, and Stephen L. Moshier, Cambridge, Mass., assignors to Listening Incorporated, Arlington, Mass., a corporation of Massachusetts Filed Sept. 27, 1968, Ser. No. 763,103 Int. Cl. Gk 11/10 US. Cl. 1s1--zs 2 Claims ABSTRACT OF THE DISCLOSURE An enlarged replica of a human pinna is provided having a highly sound reflective surface. The reproduction is mounted to an acoustic isolator which, when placed over the users ear, is effective to preclude all external sound from reaching the users ear except that sound which passes through the enlarged reproduction. The device may be used to practice our discovery that the size of the pinna does not adversely affect the character of an impinging sound wave so as to render the sound location unintelligible.

SUMMARY OF THE INVENTION Our invention makes use of our discovery that enlargement of a reproduction of a human pinna does not affect the ability of the user to localize sound. The human pinna, because of its shape and contour, collects and reflects impinging sounds in a manner that introduces delays to the impinging sound and thus causes a reverberatory effect by which the same sound repeatedly arrives at the eardrum at dilferent times. The human brain is able to localize (i.e. determine the direction of) an impinging sound by noting these delays and auto-correlating them. We have discovered that although enlargement of the pinna may increase the delay interval imparted to the impinging sound and thus accentuate this reverberatory effect, the human brain is capable of performing and does perform the same auto-correlation on the received sound so that the location of the sound source is not indeterminate as had been anticipated.

The physical embodiment of our invention includes at least one enlarged replica of a human pinna having a highly sound reflective surface. The replica is mounted to an acoustic isolator which when placed over the users ear, serves to exclude substantially all external sound, except that passing through the enlarged pinna, from reaching the users ear. A sound channel is provided to communicate the sound through the pinna and acoustic isolator to the user. This sound channel is shaped to provide an acoustic impedance match.

One of the objects of our invention is to provide a device for mechanically amplifying sound waves between the source of the sound and the listeners ear.

Another object of our invention is to provide a mechanical transducer capable of mechanically amplifying sound without distortion so that the sound may remain intelligible to the listener.

A further object of our invention is to provide a mechanical transducer of the type described which is adapted to enable the listener to localize the direction from which the sound is transmitted.

Our invention will now be described in greater detail with reference to the accompanying drawing which illustrates the construction of our invention, and from which other objects and modifications of our invention will be apparent.

In the accompanying drawing, FIG. 1 is a sketch showing the acoustic transducers of our invention in use mounted on the users head; and

3,513,937 Patented May 26, 1970 "ice SPECIFIC DESCRIPTION As shown in FIG. 1 our invention includes a pair of acoustic horns 10 that are enlarged scale replicas of the pinna portion of human outer ears. Each of the pinnae is mounted to a flexible headband 12 to enable the listener to place securely the device about his head with the pinnae 10 over his ears. Each of the pinnae 10' is provided with a canal or sound channel 14 (see FIG. 2), the outward end of which is in acoustic communication with the concha of the pinna 10 to couple acoustically the listeners real car with the outwardly facing, sound-receiving pinnae 10. To properly utilize my invention it is important that the only sound transmitted to the listeners real car he that which is collected by the enlarged pinnae 10. To this end each pinna 10 is mounted to an acoustic isolator 16 having a relatively soft, large-area circumaural pad 18 which engages the side of the listeners head and provides an efficient acoustic seal to exclude sound waves, other than those channeled through the enlarged pinna 10, from reaching the listeners ear.

Although in theory the most efficient coupling between the enlarged pinna 10 and the listeners ear canal would consist of an acoustic exponential horn leading from the sound channel 14 to the listeners real ear canal and means providing an airtight seal at both ends of the horn, practical considerations require that our device be capable of use by persons having a wide variety of ear canal sizes and cross sectional shapes. Such an ideal connection would also preferably be variable in length soas to be properly fitted to persons having varying distances between the ear canal and the side of the head.

One feature of our invention is intended to provide a coupling to the human ear that is highly efficient and is capable of use by subjects of different sizes. To this end the inwardly directed throat of the sound channel 14 is flared in the region indicated by the reference character 20 and is made contiguous with the inner ear-engaging surface 22. Thus when the device is placed on the users head with the circumaural pads 18 circumscribing the listeners ear, the inner surface 22 will lightly engage the real pinna of the user and the flared portion 20 of the sound channel 14 may be positioned substantially in registry with the listeners concha so as to be coupled acoustically to the concha, the cavity just outside the ear canal. The flared opening 20 of the pinna reproduction is approximately of the same size as the users concha in order to obtain an acoustic impedance match.

As an illustrative example of our invention scale replicas of human ears three times actual as normal size were constructed in the manner described below. A triple scale replica of a human ear was carved from clay and a negative rubber mold was made from the clay master. Fabrication of the negative mold from rubber or other flexible material is desirable because the extensive undercuts and narrow convolutions present in the shape of the pinna require a flexible mold that may be non-destructively separated from the finished casting so as to be reusable.

It is desirable to fabricate our device from lightweight materials so that its use for an extended period of time will not cause discomfort or otherwise hinder the listener. Accordingly we have found that constructing the enlarged reproductions from expanded plastic foam is suitable for this purpose. When manufacturing the pinna 10 of expanded plastic foam it is desirable to support the rubber negative mold described above in a complementary supporting mold to enable the negative mold to resist de- 3 formation as a result of the forces created by the expanding foam.

The plastic foam exhibits relatively inferior acoustic reflective properties in that it absorbs a significant portion of the sound and its effect is to muflle the sound rather than to amplify it. To render the reproduced pinna more reflective and yet retain its lightweight characteristics we have coated the plastic foam pinna with a rigid epoxy resin to produce a highly reflective coating. The foam used in making the ears was Vultafoam No. RUR-17843 a product of General Latex & Chemical Corp., 666 main Street, Cambridge, Mass.

Our enlarged pinna reproduction functions in the same manner as a real pinna to collect and reflect an arriving sound wave front. When the sound wave impinges on the pinna it is reflected by the convolutions of the pinna to impart a reverberatory quality to the sound before it reaches the eardrum. This reverberatory effect is generated by reason of the fact that as the sound Wave is reflected by the convolutions of the pinna the sound wave is directed to the eardrum along a number of paths of varying length. Thus the eardrum receives the same sound wave at various times determined by the length of the various paths leading to the eardrum. From this it is apparent that as a sound source is moved from one location to another the length of the path traveled by the sound wave as it is reflected from the pinna to the eardrum will change accordingly. Additionally the changed position of the sound source with respect to the pinna results in a change in attenuation of the sound wave by the pinna.

It has been generally believed that the brain interprets the phased and attenuated sound wave by performing an auto-correlation function so that the direction of the sound may be determined. This localization capability has been verified by tests which have shown that a person who is totally deaf in one ear can still localize sounds although the localization is not as accurate as if he had two ears.

It has also been the prevalent belief that by varying the size of the pinna the time delays imparted to the reflected phases of the sound waves would be different and this would make the localization function unworkable. For example if a sound source traveled slowly by a transducer which was a one to one scale reproduction of a human pinna it would appear to the listener that the sound source went slowly by the ear. However, it was believed that with an enlarged scale pinna the sound source would appear to oscillate back and forth, and to move more rapidly than its actual velocity. For this reason, when a reproduction of a human pinna was used for localization of sound underwater the pinna was enlarged to scale four and one half times actual size in order to maintain the same ratio of pinna size to velocity of sound in water as to velocity of sound in air. It was believed that if the same ratio were not maintained the eardrum receiving the reflected phases of the sound wave having different and unfamiliar delays would be unintelligible to the brain. This belief was supported in part by the fact that the distortion of the pinna, as by handling, causes a distortion of the localization and character of the sound.

We have discovered that enlarging the scale of the pinnae does not result in the above difliculties but instead results in the sounds being intelligible and of an increased intensity. In addition to coupling the output of the enlarged pinna reproduction directly to a human ear, an acoustically isolated microphone may be inserted in the canal 14. The microphone should be isolated so that it is respective only to sound from the pinna reproduction. The electrical signalfrom the microphone may then be supplied to a headphone or two headphones if two pinnae are used or processed in any other desired manner.

It will be apparent from the foregoing that we have provided an improved acoustic transducer for use by human listeners either directly or by means of a microphone and headset. Our transducer includes a lightweight enlarged reproduction of a human pinna having a highly sound reflecting surface coupled to an acoustic isolator. Our transducer not only increases the intensity of the sound perceived by the user but also enables him to localize its direction.

It will thus be seen that the objects set forth above, among those made apparent fromthe preceding description, are efliciently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A directional acoustic transducer having sound amplifying characteristics comprising:

a pair of enlarged reproductions of a representative normal adult human pinna, the scale of enlargement being approximately three times normal, each of said reproduced pinnae being constructed of a light- .weight material provided with a highly sound reflective outer surface and a sound channel formed therein, one end of each of said sound channels being in acoustic communication with the concha portion of its associated pinna reproduction,

the other end of each of said sound channels being adapted to be disposed adjacent to and in registry with the concha of the listeners real ear to enable sound to be transmitted from the reproduced pinnae through the sound channel to the listeners ear; a headband adapted to fit about the listeners head, said reproduced pinnae being secured to said headband so that said headband may be positioned on the listeners head with said other sound channels of said reproduced pinnae in registry with the concha of the listeners real ears; and

a relatively soft circumaural pad mounted to each of said reproduced pinnae, and circumscribing said other end of said sound channel, said pads being dimensioned as to circumscribe the listeners ear and engage the side of the listeners head whereby the listeners ears will be acoustically isolated from all external sounds except those passing through the sound channel.

2. A transducer as set forth in claim 1 wherein said lightweight porous material is plastic foam and said outer surface is a relatively hard epoxy resin.

References Cited UNITED STATES PATENTS 1,761,666 6/ 1930 Hinternesch 181-25 2,643,729 6/ 1953 McCracken 181--34 3,073,410 1/ 1963 Gongoll et al 18123 3,139,150 6/1964 Weil 181-25 FOREIGN PATENTS 1,090,897 10/ 1954 France.

832,113 4/1960 Great Britain.

STEPHEN I. TOMSKY, Primary Examiner US. Cl. X.R. l8126 

